This invention relates to input oscillators used to track the synchronizing information contained in the output signal of a video tape recorder (VTR). The clock pulses produced by the oscillator are then used to sample the output signal from the video tape recorder, and write it into the storage in the time base corrector. The invention is an improvement on the input oscillator circuit described in our co-pending U.S. Pat. Nos. 558,091 and 558,090, under the title Improvements to Digital Time Base Correctors for Television Equipment.
A first improvement relates to that part of the circuit used for combining the synchronizing information derived from the incoming horizontal synchronizing pulse and from the zero crossings of the colour sub carrier burst. It is required to produce a starting pulse for the triggered oscillator from these two pieces of synchronizing information. The horizontal synchronizing pulse is used as a coarse timing reference while the colour sub carrier burst is used for fine timing. In a simple system disclosed in our copending applications referred to above, a monostable is triggered by the leading edge of the incoming horizontal synchronizing pulse. This monostable produces an output pulse whose back edge coincides approximately in time with the centre of the colour burst after the burst has passed through a band pass filter of 0.5 MHz band width. The next zero crossing of the filtered sub carrier burst occurring after the trailing edge of the pulse produced by the monostable is then used as the timing reference to start the triggered oscillator.
One of the problems with this simple system occurs because there is no defined phase relationship between the sub carrier burst phase and the horizontal synchronizing pulse. Therefore at certain timing relationships between the burst zero crossings and the horizontal synchronizing pulse leading edge, there is an ambiguity regarding which sub carrier zero crossing occurs after the monostable pulse has ended. Because in a practical system there will be noise present on the synchronizing information, the oscillator start pulse produced may jitter in time by one sub carrier period.